PRIME is a little sleeker than the Jawbone 2 (but not by much) and comes with a re-designed "new fit" earbud that allows you to wear it without an earloop (the piece that goes over your ear). The earbud that was attached in the box happened to fit me perfectly, but they also include three traditional earbuds, three new fit earbuds and one earloop in the box - which should fit most ears.
Although you don't need to use it with an earloop, I prefer to. It's partially out of habit (I always wore one with the Jawbone 2) and partially for safety. No one wants to lose their $130 Bluetooth headset, and wearing the earloop gives me a little extra piece of mind.
Sound-wise it's amazing. Jawbone uses a proprietary noise cancellation technology called NoiseAssassin 2.0 that blocks out almost all ambient noise, including wind noise while driving, construction equipment, loud concerts, noisy bars, you name it.
The new NoiseAssasin 2.0 technology is really what sets Jawbone apart. Of the dozen or so Bluetooth headsets that I've used, there's simply no comparison to Jawbone. Check out the noise cancellation demonstration video on the Jawbone site, it was recorded using a standard Bluetooth phone over a national network with no modifications to the sound quality.
Jawbone PRIME comes in several colors (including green, red, yellow and purple) which makes it stand out from all the black/silver headsets out there. I prefer a color (purple actually) over black because it's easier to find in a dark bag. If you're more of a traditional type you can choose from black, coffee or platinum.
Jawbone is best Bluetooth headset on the planet because its noise cancellation is second to none. If I had to complain about one thing, it would be the proprietary charger. If Jawbone included a standard mini-USB port and cable it would be true Bluetooth nirvana.
What Bluetooth headset are you using?